This invention relates to apparatus for detecting articles and substances concealed on a person.
Various techniques are used to detect concealed articles and substances on people, such as at airports. One familiar technique is the walk-through metal detector presently used at all major airports to detect concealed metal articles, such as guns or knives, carried by passengers. These work effectively and cause little delay but suffer from the disadvantage that they are unable to detect non-metal objects such as ceramic knives, explosives or narcotics. Another form of equipment in present use, but on a much smaller scale, is the trace chemical detector. In one form this is incorporated into a walk-through portal having multiple air jets directed over the body of the passenger and having suction inlets located to collect trace chemicals dislodged by the air jets. The collected air and trace chemicals is supplied to a chemical detector, which may take various different forms but typically might include an ion mobility spectrometer (IMS) arranged to respond to the presence of selected chemicals. Trace chemical detectors are available from Smiths Detection. Although trace chemical detectors can work effectively, they are relatively expensive and the response time is slower than metal detectors. Furthermore, they cannot detect knives or the like.
A further form of equipment has been proposed, but has not yet been used extensively, namely millimeter wave imaging. This uses radiation in the range of about 10 GHz to 400 GHz, usually around 93 GHz, which passes substantially unimpeded through clothing and thereby enables articles concealed beneath clothing to be detected. There are different techniques employing millimeter waves, such as described in U.S. Pat. No. 4,901,084, which employs polarized coherent radiation, or as described in U.S. Pat. No. 6,353,224, which employs a source of incoherent radiation intended to replicate normal background radiation in the open air. The advantage of millimeter wave imaging is that it can detect non-metal objects such as ceramic knives and explosives carried beneath clothing. The problem with millimeter wave imaging is that the radiation does not penetrate most shoes and boots. Also it can be difficult to detect thin sheets of explosives.